This is something I've had to deal with a bit lately, with all my international travel. Is seeing if my phone will work in another country. I recently took two separate trips to Beijing, China. The first time, I used the Nexus 6. I was only able to get GPRS. But the second time I used a Meizu MX4 (the Chinese versions) and was able to get 4G LTE. But with a site like willmyphonework.net, I'd be able to see which one would work better in Beijing before I leave. Which is going to be a huge help.
To use the site all you do is choose your phone's manufacturer, then you'll choose the model and model number. Since there are variants of each model, you'll want to be sure you choose the right model number or you won't get accurate info. This information can usually be found under Settings > About Phone. After that you'll choose the country you're going to and the carrier you want to check on.
Now since I'm going to Barcelona at the end of the month for Mobile World Congress, I'm checking to see which of the many phones I have will work the best in Barcelona and on which carrier. So if you look at the screenshot above, you'll see that the Droid Turbo does work on Vodafone in Spain, and supports 4G, as well as Orange. But you don't have to use this site just for traveling. If you're looking to buy an unlocked phone and want to see if it'll work on your carrier, this tool works great for that as well. Just for fun, I checked the Droid Turbo on T-Mobile, which it does support 3G, HSPA+, and 4G LTE which I already knew from putting my T-Mobile SIM card in there.
It's a really great tool for those that travel a lot and those that buy or use a lot of unlocked phones, like myself.